1. Field of the Invention
The present invention relates generally to an Ethernet passive optical network (EPON), and in particular, to an effective multipoint gating control method between an optical line termination (OLT) and a plurality of optical network units (ONUs) of an EPON.
2. Description of the Related Art
Standardization of medium access control (MAC) technology for Gigabit Ethernet and asynchronous transfer mode-passive optical network (ATM-PON) has been completed recently, and the contents thereof are disclosed in IEEE 802.3z and ITU-T G.983.1. In an ATM-PON, upstream and downstream transmissions are performed using a frame, which consist of a group of ATM cells having a predetermined size. In a tree-type PON, an OLT selectively inserts downstream cells to a frame to be distributed to the respective ONUs.
FIG. 1 is a block diagram illustrating a physical network structure of a general passive optical network. FIG. 1 shows an example in which three ONUs 110-1 to 110-3 are connected to one OLT 100. At least one end user (or network device) 120-1 to 120-3 is connected to each of the ONUs 110-1 to 110-3. Data 131-1 to 133-1 transmitted by the end users 120-1 to 120-3 is delivered to the OLT 100 via the ONUs 110-1 to 110-3.
In operation, ONU's data is accessed by time division multiplexing (TDM) for upstream transmission. An optical distribution network (ODN), which is a passive element, prevents data collision through a so-called ranging method. In other words, during upstream transmission, data from the ONUs 110-1 to 110-3 is multiplexed before being transmitted to the OLT 100. During downstream transmission, the ONUs 110-1 to 110-3 receiving data broadcasted by the OLT 100 select only its own data from the received data. To this end, upstream and downstream frames include a field capable of exchanging messages at the specified periods, which assigned in a private ATM cell or a general ATM cell. With the development of Internet technology, a subscriber side now requires more bandwidth, so an end-to-end transmission is typically achieved with Gigabit Ethernet technology, which provides relatively cheaper equipments and higher bandwidth over the ATM technology, which requires segmentation of IP packet and has a limited bandwidth. Therefore, Ethernet technology is ideal for the PON structure instead of ATM technology. Standardization of such an EPON is now under way in IEEE 802.3ah in the name of EFM (Ethernet in the First Mile).
Such an EPON, unlike the conventional Ethernet-based network, is characterized by having a point-to-multipoint structure instead of a point-to-point (PTP) structure. Therefore, Draft v1.0 of IEEE 802.3ah has introduced a virtual MAC (vMAC) structure so that an OLT which is a master can efficiently manage respective ONUs.
FIG. 2 is a diagram illustrating a vMAC structure for an OLT in the EPON standard. As shown in FIG. 2, unlike the existing MAC, the illustrated MAC is divided into several virtual MACs, and the virtual MACs are mapped to respective ONUs. That is, upon receiving a frame, a reconcile sublayer (RS) analyzes a logical link identifier (LLID), splits the received frame to a vMAC associated with the LLID, and delivers the split frame to an upper layer. In actual implementation, vMACs can be optionally implemented with one MAC.
In this case, an optical multipoint (OMP) function block for implementing a multipoint control protocol (MPCP) of an EPON as well as a function of managing and controlling a plurality of MACs is achieved in the MAC control layer. Therefore, this is called “multipoint MAC control” in IEEE 802.3ah. The OMP function block includes a Flow control function performed in the existing MAC control, and such MPCH functions as Gate processing, Report processing, and Discovery processing functions.
Meanwhile, in EPON standardization which is now under way in IEEE 802.3ah, a control block is required which informs the respective logical or physical MACs for the transmission of the frames in order for several MAC clients to send frames to one PHY, or physical port, via their logical of physical MACs. If several logical or physical MACs simultaneously transmit frames, collision may occur in the PHY, or physical port, shared by the MACs.
In order to prevent such collision between MACs, the ongoing IEEE 802.3ah EPON standardization has proposed Muxing Control for controlling the collision. Its fundamental operational principle is to provide a transmission enable signal (transmitEnable) to respective logical or physical MACs, to control when and how long the MACs can transmit fames.
FIG. 3 is a diagram illustrating a case in which several MAC clients transmit frames to one physical port. As shown in FIG. 3, a multiplexing control block 230 can control (or inform) several logical or physical MACs when and how long they can transmit frames. However, while a logical or physical MAC 204 is transmitting a frame in response to its transmission enable signal (transmitEnable) being turned On, if the transmission enable signal transmitEnable to the MAC 204 is turned Off and a transmission enable signal (transmitEnable) to a logical or physical MAC 214 is turned On, collision occurs between the frame being transmitted by the MAC 204 and a frame newly transmitted by the MAC 214. In order to prevent the collision, frame transmission by the MAC 204 must be immediately suspended. As a result, in the conventional standard technology, frame collision or frame loss may occur during the frame transmission.
In this structure, since MAC clients as well as MACs are separated, there is a demand for a mechanism capable of transmitting only one frame at a time during frame transmission from a MAC client to a MAC. However, the current Draft provides no definition on it.